1. Field of the Invention
The present invention relates to an output buffer circuit installed in a semiconductor integrated circuit device for converting logical information processed by the device into an appropriate logic signal to deliver the converted signal to an external unit, and in particular, to an output buffer circuit including a pre-emphasis function to appropriately emphasize a waveform on a transmission side in advance according to attenuation of a signal through a transmission line.
2. Description of the Related Art
It has been a common practice to employ a circuit including a pre-emphasis function as an output circuit, particularly, for long-distance, low-voltage, high-speed, i.e., high-frequency transmission.
The output buffer circuit of this kind generally includes a current-mode circuit handling a signal represented by a current, but it is not advantageous to operate the current-mode circuit with a low power-source voltage due to structure thereof.
However, the recent development of the fine machining technology for semiconductor integrated circuits has lowered the operation voltage required to operate the circuits and hence has reduced power consumption thereof. It is therefore required to operate the semiconductor integrated circuits at a higher speed with a lower power-source voltage.
The conventional techniques to meet the requirements include those described in Japanese Patent Application Laid-Open Ser. No. 2000-68816 entitled “Output Buffer Circuit for Transmitting Digital Signal via Pre-emphasis Transmission Line” and Japanese Patent Application Laid-Open Ser. No. 2002-94365 entitled “Output Buffer Circuit”.
Particularly, the technique of Japanese Patent Application Laid-Open Ser. No. 2002-94365 drives an output circuit with a low power-source voltage and shortens a period of time of propagation delay from an input port to an output port to thereby implement a high-speed operation of the output circuit.
However, the invention of Japanese Patent Application Laid-Open Ser. No. 2002-94365 is attended with a problem that the consumption power increases when a de-emphasis state continues.